In higher organisms, any given cell expresses only a fraction of the total number of genes present in its genome. The small fraction of the total number of genes that is expressed determine the life processes carried out by the cell, e.g. development and differentiation, homeostasis, response to insults, cell cycle regulation, aging, apoptosis, and the like. Alterations in gene expression decide the course of normal cell development and the appearance of diseased states, such as cancer. Because the choice of which genes are expressed has such a profound effect on the nature of any given cell, methods of analyzing gene expression are of critical import to basic molecular biological research. Identification of differentially-expressed genes can provide a key to diagnosis, prognosis and treatment of a variety of diseases or condition states in animals, including humans, and plants. Additionally, these methods can be used to identify differentially-expressed sequences due to changes in gene expression level associated with predisposition to disease, influence of external treatments, factors or infectious agents. Identification of such genes helps in development of new drugs and diagnostic methods for treating or preventing the occurrence of such diseases.
One way of analyzing gene expression in a particular cell is to perform differential gene expression assays, in which the expression of genes in different cells is compared and any discrepancies in expression are identified, where the presence of discrepancies indicates a difference in the classes of genes expressed in the cells being compared.
One method currently employed to identify differentially expressed genes begins with the generation of cDNA "targets" obtained from analogous cells, tissues or organs of a healthy and diseased organism. The cDNA targets are then hybridized to a set of target nucleic acid "probe" fragments immobilized on membrane. Differences between the resultant hybridization patterns are then detected and related to differences in gene expression in the two sources. In this procedure the number of analyzed gene-specific probes can reach several hundred thousand.
Modifications have been made to the above basic method in order to obtain improved results. These modifications include replacement of the traditional radioactive labeling procedure of the target nucleic acid sequences with nonisotopic labels, mainly fluorescent labels. Other modifications have focused on improved methods of immobilization of an array of the probe nucleic acids to surfaces of a variety of solid supports.
Despite the promise of analysis of differential expression using arrays of probes on solid supports, there is a continuing need for improvement of the methods currently employed by researchers. In current methods, hybridization of "target" to "probe" is slow. Furthermore, a number of additional events such as competitive hybridization events between distinct target sequences, nonspecific binding between "target" and "probe," and formation of secondary structures in target sequences can occur which adversely effect the results.
Accordingly, there is continued interest in the development of new methods of analyzing differential gene expression, where such methods provide for fast hybridization and high specificity of binding of "targets" to "probes."
Relevant Literature
Patents of interest include: EP 0 328 829 B1 and U.S. Pat. Nos. 5,468,613; 5,580,726; 5,599,672; 5,512,462; 5,162,209 and 5,162,209. Methods of analyzing differential gene expression are also described in Maniatis, et al., Molecular Cloning, A Laboratory Manual, (Cold Spring Harbor Press, Cold Spring Harbor, N.Y.)(1989); Nucleic Acid Hybridization, A Practical Approach (Hames, B. D., and Higgins, S. J. eds, IRL Press, Oxford)(1985); WO 95/21944; Chalifour, et al., Anal. Biochem. (1994) 216: 299-304; Nguyen et al., Genomics (1995) 29: 207-216; Pietu et al., Genome Res. (1996) 6: 492-503; and Zhao et al., Gene (1995) 166: 207-213.
Use of non-isotopic labels in methods of differential gene expression analysis are described in: Schena et al. Science (1995) 270: 467-470; Schena et al., Proc. Natl. Acad. Sci. USA (1996) 93: 10614-10619; DeRisi et al., Nature Genet. (1996) 14: 457-460; and Lockhart et al., Nature Biotechnol. (1996) 14: 1675-1680.
Methods of stably associating probes to the surface of substrates are described in: Hermanson, et al. Immobilized Affinity Ligand Techniques, Academic Press, (1992); WO 89/11548; European Patent No. 0 281 390 B1; WO 88/01302; European Patent Application No. 0392546; U.S. Pat. No. 5,436,327; U.S. Pat. No. 5,445,934.
Methods of improving hybridization of target to substrate surface associated probe are described in: EP 0 318 245 B1 (solution hybridization of probe to target followed by binding of hybridization complex to surface of substrate); Lockhart et al., Nature Biotechnol. (1996) 14: 1675-1680, EP 0 328 829 B1 (preamplification of target DNA/RNA); Maniatis et al., Molecular Cloning, A Laboratory Manual (Cold Spring Harbor Press, Cold Spring Harbor, N.Y., (1989), Nucleic Acid Hybridization, A Practical Approach (Hames, B. D., and Higgins, S. J. eds. IRL Press, Oxford) (1985), EP 0 229 442 (addition of an inert polymers such as dextran sulfate); U.S. Pat. No. 5,387,510, EP 0 318 245 B1 (use of "helper" oligonucleotides which reorder secondary and tertiary structure of target polynucleotide); WO 89/11548 (attaching probes to surface of substrate through long spacer arms).
Methods of improving specificity of hybridization are described in: U.S. Pat. Nos. 5,449,603 & 5,547,843 (use of single stranded nucleic acid binding protein); U.S. Pat. Nos. 4,888,274 & 5,223,414, EP 0 481 065 B1 (use of RecA protein-coated nucleoprotein target molecules); Khrapko et al., FEBS Lett. (1989) 256: 118-122 and U.S. Pat. No. 5,503,980 (continuous stacking interaction between short oligonucleotides of target and probe molecules, followed by enzymatic ligation step); and U.S. Pat. No. 5,434,047 (use of non-target probe which hybridized with non-target nucleic acid).